1. Field of the Invention
The present invention is generally in the field of electrical circuits and systems. More specifically, the present invention is in the field of energy storage circuits and systems.
2. Background Art
The storage and on-demand delivery of electrical energy is increasingly important as the transition from fossil fuels to renewable and other so called “green” energy alternatives continues. Electric vehicles and gas/electric hybrid automobiles, for example, typically utilize a battery pack of secondary battery cells that are alternately discharged and charged as the vehicle is operated, and that may be fully recharged when the vehicle is not in use. Such secondary battery packs may constitute a substantial portion of the cost of an electric or gas/electric hybrid vehicle, and the performance of the vehicle battery pack, and in particular its longevity, may significantly influence consumer willingness to invest in the initially costlier vehicle purchase price.
Conventional battery based power systems, such as the secondary battery packs used in electric and gas/electric hybrid vehicles, for example, usually comprise a collection of secondary battery cells that are nominally identical, e.g., battery cells sharing the same chemistry and providing substantially the same cell voltage, connected in series in order to provide the relatively high voltages required for vehicle operation, e.g., several hundred volts. As a result, the type of secondary battery cell conventionally selected for use often represents a compromise among several competing factors. For example, although a battery pack may be called upon for heavy power discharge during vehicle acceleration, for sustained power discharge during normal driving, and for rapid power uptake during regenerative braking, no single secondary battery cell type may provide optimum performance for all three operations. Moreover, considerations such as battery pack cost, weight, and anticipated longevity must also be taken into account when selecting the type of secondary battery best suited for use.
In addition to representing a compromise that is likely to be less than optimal for any one of multiple performance objectives, the conventional approach to powering vehicles electrically may prematurely reduce the usable life of the battery pack. For example, as mentioned above, the conventional approach to providing electric power to a vehicle may use the same battery pack as a power source for acceleration and as a power storage reservoir during energy regenerating operations, such as regenerative braking. However, because operation of a vehicle is typically a highly dynamic process, acceleration and braking can be expected to alternate often, resulting in frequent charge cycling of the battery pack, thereby undesirably shortening battery life.
Thus, there is a need to overcome the drawbacks and deficiencies in the art by providing an electric energy storage system capable of providing substantially optimized power transfer in response to a broad spectrum of performance requirements, while also advantageously improving battery or other energy storage cell longevity.